The present invention relates to the field of endodontic instruments which are adapted for use in performing root canal therapy on teeth, and which are characterized by high flexibility and high resistance to torsional breakage.
Root canal therapy is a well-known procedure wherein the crown of a diseased tooth is opened so as to permit the canal to be cleaned and then filled. More particularly, after opening the crown, a series of very delicate, flexible instruments which are commonly called files are used to clean out and shape the root canal. Each file is rotated either manually or by a powered dental handpiece and reciprocated in the canal by the clinician, and files of increasingly larger diameter are used in sequence, to achieve the desired cleaning and shaping. When the canal is thus prepared, it is solidly filled with a filling material, which typically comprises a waxy, rubbery compound known as gutta percha. In one procedure, the gutta percha is positioned on an instrument called a compactor, and the coated compactor is inserted into the prepared canal and rotated and reciprocated to compact the gutta percha therein. The clinician then fills the tooth above the gutta percha with a protective cement, and lastly, fits a crown to the tooth.
Endodontic files of the described type are commonly supplied to the clinician in kits which comprise several files of increasing diameter. In particular, and in accordance with ANSI/ADA Specification No. 28-1988, files are provided in diameters which range from 0.08 mm at the tip (size 08) to 1.40 mm at the tip (size 140), and the files are provided in kits which contain a number of files of increasing diameter so that the files from a particular kit may be used in sequence by the clinician in accordance with the requirements of the particular canal being cleaned. Also, in the conventional kits, the working length of each file in the kit is tapered, at an included angle of between about 1/2 and 4 degrees, and with the tapers of all of the files in the kit being the same.
As is well-known by clinicians, procedural errors and accidents occasionally occur during root canal therapy, one of the most serious being the breakage of a file in the canal, since the remaining file fragment often cannot be removed from the canal. Such breakage can result from the rapid flexing of the file while it is being rotated in a curved canal. In addition, since the tapered configuration of the canal which is being formed will necessarily match the taper of the file, a locking interconnection can be formed between the wall of the canal and the surface of the rotating file along the full length of the file, and this locking interconnection results in a frictional binding which can severely stress the file and result in its breakage. This problem is particularly pronounced when the file is rapidly rotated in the canal by a powered dental handpiece, although the problem is also believed to be present when the file is rotated by hand.
Recently, endodontic files composed of a nickel-titanium alloy have been introduced, which provide a high degree of flexibility in both bending and torsion, and superior resistance to breakage, as compared to stainless steel instruments. In this regard, reference is made to the article entitled "An Initial Investigation of the Bending and the Torsional Properties of Nitinol Root Canal Files", Journal of Endodontics, Volume 14, No. 7, July 1988, at pages 346-351, and to copending U.S. application Ser. No. 08/271,645, now U.S. Pat. No. 5,464,362.
While the use of nickel-titanium instruments is seen to significantly reduce the breakage problem, it is an object of the present invention to provide a kit of tapered endodontic instruments which are configured to avoid a locking interconnection between the wall of the tapered canal being formed and the tapered instrument, when the instrument is rapidly rotated in the canal by a machine driven handpiece, to thereby avoid excessive stress being placed on the instrument and so as to further minimize the risk of breakage of the instrument in the canal.